AR-News: (DC) Wolves are rebalancing yellowstone ecosystem
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Tue Oct 28 13:32:07 EST 2003
CORVALLIS, Ore. - The reintroduction of wolves into Yellowstone National Park
may be the key to maintaining groves of cottonwood trees that were well on
their way to localized extinction, and is working to rebalance a stream
ecosystem in the park for the first time in seven decades, Oregon State University
scientists say in two new studies.
The data show a clear and remarkable linkage between the presence of wolves
and the health of an entire streamside ecosystem, including two species of
cottonwoods and the myriad of roles they play in erosion control, stream health,
and nurturing diverse plant and animal life.
The findings of these studies were recently published in Ecological
Applications, a journal of the Ecological Society of America, and the journal Forest
Ecology and Management.
"In one portion of the elk's winter range along the Lamar River of
Yellowstone National Park, we found that there were thousands of small cottonwood
seedlings," said Robert Beschta, professor emeritus in the College of Forestry at
OSU and an expert on streams and riparian systems. "There should also have been
hundreds of young trees, but there were none. Long-term elk browsing had been
preventing any seedlings from getting taller."
That pattern was common throughout the study area - lots of seedlings in
combination with large cottonwood trees generally more than 70 years old, but
little or nothing in between.
Young cottonwoods, willows, and other streamside woody species are a
preferred food for browsing elk during the harsh winters in northern Yellowstone, when
much of the other forage is buried under snow. But when packs of wolves
historically roamed the area, food was not the only consideration for elk, which
had to be very careful and apparently avoided browsing in high-risk areas with
low visibility or escape barriers.
Wolves were systematically killed in the Yellowstone region and many other
areas of the West beginning in the late 1800s. A concentrated effort between
1914 and 1926 finished the job - the last known wolf pack disappeared in 1926.
"I considered a variety of potential reasons that might explain the
historical decline of cottonwoods that began in the 1920s and have continued up to the
last couple of years," said Beschta. "I looked at climate change, lack of
floods, fire suppression, natural stand dynamics, and numbers of elk. But none of
those factors really explained the problem. "Ultimately, it became clear that
wolves were the answer."
While elk populations fluctuated over the decades when wolves were absent,
browsing behavior appears to represent an important factor related to streamside
impacts. With no fear of wolves, the elk could graze anywhere they liked and
for decades have been able to kill, by browsing, nearly all the young
cottonwoods. Other streamside species such as willows and berry-producing shrubs also
That in turn began to play havoc with an entire streamside ecosystem and
associated wildlife, including birds, insects, fish and others. Trees and shrubs
were lost that could have helped control stream erosion. Food webs broke down.
"Before the wolves came back, it was pretty clear that in some areas we were
heading towards an outright extinction of cottonwoods," Beschta said.
Now, with the recent reintroduction of wolves back into Yellowstone in 1995,
streamside shrubs and cottonwoods within the Lamar Valley are beginning to
become more prevalent and taller, and were the focus of a second study in the
same area. That study outlines how the fear of attack by wolves apparently
prevents browsing elk from eating young cottonwood and willows in some streamside
With the renewed presence of wolves, young cottonwoods and willows have been
growing taller each year over the last four years on "high-risk" sites, where
elk apparently feel vulnerable due to terrain or other conditions that might
prevent escape. In contrast, on "low-risk" sites, they are still being browsed
by elk and show little increase in height.
"In one case where a gully formed an escape barrier for elk, the tree height
went up proportionally as the gully deepened and formed an increasing barrier
to escape," said William Ripple, a professor with the College of Forestry at
OSU. "Where the fear factor of wolves is high, the young trees and willows are
doing much better and growing taller."
Traditionally, "keystone" predators such as wolves were known to influence
the population of other animals that they preyed on directly, such as elk or
antelope. What researchers are now coming to better understand is the "trophic
effect," or cascade of changes that can take place in an ecosystem when an
important part is removed, Ripple said.
The comparatively pristine conditions of a national park allowed this type of
research to make "cause and effect" studies more feasible, the scientists
"The removal of wolves for 70 years - and then their return - actually set
the stage for a scientific experiment with fairly compelling results," Beschta
In a larger context, the studies also raise valid questions about other
complex and poorly understood interactions between plants, animals, and wildlife in
disturbed ecosystems across much of the American West, and perhaps elsewhere
in the world, the scientists say. In some areas of the West, the disappearance
of up to 90 percent of the aspen trees has been documented - another species
of plant that is also highly vulnerable to animal browsing when it is young.
"The last period when aspen trees in Yellowstone escaped the effects of elk
browsing to generate trees into the forest overstory was the 1920s," Ripple
said, "which is also when wolves were removed from the park."
But in at least one place - America's first national park - there is now
cause for hope. While it is too early to confirm the widespread recovery of
cottonwoods and willows, the reintroduction of wolves appears to have put a stop to
major declines in the survival of these plants, the researchers found.
"One point that should not be missed is this is actually great news for the
potential recovery of cottonwood trees and mature willows in Yellowstone
National Park," Ripple said. "We now have a pretty good idea why they were in
decline and the return of wolves should help pave the way for their recovery.
"Even though it may take a very long time, for a change it looks like we're
headed in the right direction."
By David Stauth, 541-737-0787
I am sometimes asked, 'Why do you spend so much of your time and money
talking about kindness to animals when there is so much cruelty to men?' I answer,
'I am working at the roots.'" ~ George T. Angell
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